Tissue-specific enhancers seem to play a major role in controlling developmentally regulated gene expression. Expression of rearranged immunoglobulin (Ig) genes introduced into both lymphoid and non-lymphoid cells has led to the identification of tissue-specific transcriptional enhancer sequences in the major intron between the J and C region of the Ig gene. We have shown that Ig promoter also contributes to tissue specific expression of mouse Ig Kappa gene. Tissue-specificity of Ig gene enhancers and promoters could be due to their interaction with some, as yet unidentified, trans-acting regulatory factors that may be present only in lymphoid cells. We have constructed special plasmid vectors containing the coding sequences of a dominant selectable marker gene that confers resistance to neomycin. These coding sequences are driven by an Ig gene promoter and the various vectors contain different Ig gene enhancers. By introducing these hybrid genes into non-lymphoid cells, we have created recipient cell clones in which the hybrid gene is stably integrated and non-functional. These cell lines can be used to directly isolate genes that code for putative trans-acting regulatory factors. Isolation of these regulatory genes will greatly enhance our ability to understand the regulation of tissue-specific genes at the molecular level.